Want to study craters? Geologists grab explosives and make some

We have video of researchers blowing things up to understand volcanic craters.

Enlarge / View from the edge of Kilbourne Hole in New Mexico—an example of a maar crater.

Scott K. Johnson

While craters cover many other bodies in the solar system, plate tectonics and weathering continually renew the Earth’s surface, preserving its youthful beauty. Still, that process doesn't happen overnight, and there are many craters to be found on our planet. Some record violent impacts with meteorites, and others formed during a variety of volcanic eruptions.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion. Measuring the size of a meteorite impact crater can provide a lot of information about the size and impact angle of the meteorite. But when it comes to maar craters, geologists have been unsure just how much information about the eruption can be gleaned from the remnant crater.

Part of the problem results from the explosion being able to occur at a range of depths. An explosion of the same size could create a very different crater at the surface depending on how deep it occurs. To complicate matters further, there can sometimes be multiple eruptions beneath the same crater.

While this has been studied in detail using numerical simulations, a group of researchers decided a lot could be gained from examining these things with actual experiments. That is to say, they opted to go outside and blow stuff up.

Obviously, they had to work on a much smaller scale—as a general rule, astronauts shouldn't be able to notice your experiment from the space station—so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across. Within the sediment, they buried charges of TNT and plastic explosive (for good measure, one assumes) in several arrangements. This seems like a good point to mention that absolutely no one should be trying this at home.

It was a pretty straight-forward setup, as Prof. Greg Valentine, a geologist at the University of Buffalo, explained to Ars. “The configurations of explosives were also very simple, and really this was intended to be a scoping exercise in preparation for more detailed and costly experimental runs in the future. However, I would say that the setup worked well beyond our expectations—despite the simplicity, we learned an incredible amount.”

A single charge goes off 50cm deep.

In the first experiment (which can be seen in the video above), one large charge was buried 50 cm below the surface. After detonation, measurements were made of the crater diameter and volume, how much material (called “ejecta”) was blasted out of the crater, and the high-speed video was used to track the process in slow motion.

In subsequent experiments, the charge was broken into three parts and detonated one at a time, to simulate multiple eruptions. The first thing the experimenters learned was that the final dimensions of the craters were pretty similar to ones caused by a single explosion. What this says is that it's easy to overestimate the size of an eruption if you assume a crater was the result of a single eruption when it was actually several. And that’s important if the area is still volcanically active, and you’re trying to figure out how big a future eruption might be.

A charge goes off within the crater generated by an earlier "eruption."

The explosion patterns also varied considerably depending on the depth of the charge. Those that were buried around 50 centimeters below the surface launched debris more than 16 meters from the crater. In the deeper blasts, on the other hand, most of the sediment was launched upward and collapsed back into the crater, partially filling it in.

The video of those different explosion types also shed light on an interesting feature seen around some maar craters—deposits resembling wind-blown dunes. While the sand and larger bits in the ejecta fell to the ground rather promptly, the finer-grained dust hung in the air, drifting more slowly. The rapid collapse of material into the crater following the deeper explosions forced the dusty air rapidly outward along the surface. (This can be seen in the second video.)

Prof. Valentine told Ars that the research group also examined cross-sections of the sediment in an around the craters, picking up a few more insights that will be reported in a separate paper. And they have plans to do more with further experiments.

“One question is, what are the effects of different configurations of explosions on crater size?” Valentine wrote. “For example, in natural maar eruptions, we know that the explosions do not always take place beneath the center of a newly-formed crater, but sometimes are off to one side, and at varying depths. A second question is, how does the composition of the ejecta deposits (in terms of the proportions of materials from different depths) reflect the processes that go on in the subsurface explosions?”

Prof. Valentine was quick to point out that these experiments have their shortcomings, such as an inability to see what’s going on beneath the surface during the explosion, and that some aspects of maar eruptions can’t be understood by playing around with a miniature crater. “But” he added, “I have to admit, while the numerical simulations are fun and important, I really had a blast doing the experiments and micro-field work!”

While this has been studied in detail using numerical simulations, a group of researchers decided a lot could be gained from examining these things with actual experiments. That is to say, they opted to go outside and blow stuff up.

Within the sediment, they buried charges of TNT and plastic explosive (for good measure, one assumes) in several arrangements.

Most likely a bad assumption. TNT and plastic explosives have different explosive velocities. TNT is around 6900 m/s while C4 is over 8000 m/s. The use of different types of explosives was probably used to simulate the different natural explosive processes that could be experienced in the creation of a maar crater. That's just conjecture on my part, however, since I haven't read the paper.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion.

so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across.

If water is part of marr crater formation, should not the simulations also have water? I suspect this would affect the after-explosion behavior of the lighter debris and it might also dampen the force of the explosion as well.

One of your closer neighbors comes running, rather angry: What the F* are you doing?You standing with your latest of all triggers: Can't you see I'm studying geology, craters to be precise... Now please stand back.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion.

so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across.

If water is part of marr crater formation, should not the simulations also have water? I suspect this would affect the after-explosion behavior of the lighter debris and it might also dampen the force of the explosion as well.

Ideally, they'd probably like to do so. As a preliminary small-scale set of experiments, however, this is a great start.

Within the sediment, they buried charges of TNT and plastic explosive (for good measure, one assumes) in several arrangements.

Most likely a bad assumption. TNT and plastic explosives have different explosive velocities. TNT is around 6900 m/s while C4 is over 8000 m/s. The use of different types of explosives was probably used to simulate the different natural explosive processes that could be experienced in the creation of a maar crater. That's just conjecture on my part, however, since I haven't read the paper.

Yes, there is certainly a very good reason for it. I was having a bit of fun.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion.

so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across.

If water is part of marr crater formation, should not the simulations also have water? I suspect this would affect the after-explosion behavior of the lighter debris and it might also dampen the force of the explosion as well.

I think the idea is that most of the water turns to steam, which more or less can be modeled as turning a large mass of a solid/liquid into a gas (pretty much exactly what explosives do). I don't think there'd be much of a dampening effect (water is often modeled as incompressible), though I'd think the geometry of the reservoir would affect the shape of the crater quite significantly.

Obviously, the numerical simulation should try to account for that. But it's probably not that necessary when using a model that involves gas generation from a fairly different material.

Maar craters, like the one pictured above, are created when fingers of magma beneath the surface of the Earth interact with groundwater, causing a violent explosion.

so they built piles of layered sand, gravel, and crushed asphalt about a meter deep and four meters across.

If water is part of marr crater formation, should not the simulations also have water? I suspect this would affect the after-explosion behavior of the lighter debris and it might also dampen the force of the explosion as well.

I think the idea is that most of the water turns to steam, which more or less can be modeled as turning a large mass of a solid/liquid into a gas (pretty much exactly what explosives do). I don't think there'd be much of a dampening effect (water is often modeled as incompressible), though I'd think the geometry of the reservoir would affect the shape of the crater quite significantly.

Yes, water is, anywhere near STP near enough an incompressible liquid. But it also has mass, a cubic mile of water weights about 9189267859538 pounds. So I would imagine that the explosion that creates the crater will have to lift (and vaporize, which takes a lot of energy) a large and heavy volume of water. This is why I think the explosion will be dampened. Also I'd expect C4 to be a bit more energetic than a steam explosion.

Much work has already been done with respect to underground nuclear explosions.

Yes. Not having read [UndergroundEffects.html] your link I know that between 1961 and 1973 a total of 27 nuclear devices were detonated underground as part of the Plowshare program. Laurence Livermore National Laboratory carried out most of the research. Project "Sedan" for example was a test using an h-bomb that left a huge open crater on the ground surface (¼ mile wide x 320' deep). They probably decided that they could easily (but not cheaply) excavate canals with underground bombs. Some other interesting Plowshare events included the first one called "Project Gnome"(1961), and was detonated 1200' down in a salt dome. After the detonation water was pumped into the large cavern created by the blast, in hopes of harnessing the steam to create electricity. Much of the steam escaped through cracks and fissures however and what was collectible at the surface was very corrosive. Five months later the temperature of the cavern had lowered to 140° F. Scientist actually entered the spherical cavern through the parallel entrance to take pictures and samples. Three other interesting shots were in underground gas fields (Gassbuggy, Rulison Test Site and Rio Blanco Test Site) where they toyed with the idea of fracking rock to make the gas more accessible. It might have worked but the gas was too radioactive to use.

By the way:If in that 1st video the explosive was buried only a foot and a half deep (50 cm) then they could have done just as much damage with a big black-powder firecracker. Triple nitrated toluene was invented by a German back in 1863 but all they could think to do with it was to use it as a yellow dye. TNT is so insensitive that it took another 40 years before they started putting it in munitions. TNT's detonation velocity is 6900 m/sec, Mach 2 or 1,542 miles/hour. 'Composition 4' is actually 91% RDX. RDX is more sensitive than TNT with a detonation velocity of 8750 m/s, has 1.5 times more explosive power than TNT and makes a very good rat poison.

Kilbourne Hole is one of those places I never thought would show up on Ars. It is a pretty cool place to be out in the middle of nowhere and watch The Core with a geology class. Poured about a half pound of sand out of my computer after that bad geology movie of the class.

Wouldn't the reason that they used TNT and Plastic Explosive be for the multiple shot crater? You'd need the first charge not to set off the second and third charges whereas for the one big boom they can just go for the cheep stuff?

Kilbourne Hole is one of those places I never thought would show up on Ars. It is a pretty cool place to be out in the middle of nowhere and watch The Core with a geology class. Poured about a half pound of sand out of my computer after that bad geology movie of the class.

Wouldn't the reason that they used TNT and Plastic Explosive be for the multiple shot crater? You'd need the first charge not to set off the second and third charges whereas for the one big boom they can just go for the cheep stuff?

It is, indeed, an awesome place. Amazing peridotite. We had to leave a little earlier than intended after being warned that a nearby rancher was likely to "chase us off with a shotgun", though...

Charges two and three weren't present when number one blew- they buried each charge right before detonation. I'm sure it had something to do with explosion characteristics, or perhaps it was an easily obtained product.

My next two questions are, is the brissance/velocity of the TNT and plastic explosives remotely related to a Maar eruption? Or is this generally more of a comparison of the effects of multiple versus singular eruption/explosion and impact of depth on the resulting ejecta/crater size and equivelance is unimportant? I'd assume the later.

My next question is...shouldn't they really be determining the impact of having objects over the sediment layers would have on the crater size and ejecta distribution. I mean, what would happen if a laptop were over a Maar eruption? Or a car? What about box of marbles? A pallet of feathers? I mean the list goes on and I think they are all VERY important scientific questions to answer.

My next two questions are, is the brissance/velocity of the TNT and plastic explosives remotely related to a Maar eruption? Or is this generally more of a comparison of the effects of multiple versus singular eruption/explosion and impact of depth on the resulting ejecta/crater size and equivelance is unimportant? I'd assume the later.

My next question is...shouldn't they really be determining the impact of having objects over the sediment layers would have on the crater size and ejecta distribution. I mean, what would happen if a laptop were over a Maar eruption? Or a car? What about box of marbles? A pallet of feathers? I mean the list goes on and I think they are all VERY important scientific questions to answer.